The present invention relates in general to heat exchangers for metallurgical furnaces and, in particular, to a recovery-type heat exchanger with reversing flow cycle specifically designed for recovering calories from furnace smoke.
The heat contained in the smoke emitted from a furnace must be recovered if the energy supplied to the furnace is to be used efficiently. The recovered energy may be used for such general purposes as in the heating of buildings or plants. This use, however, is seasonal and is limited to cold weather applications. The annual recovery rate is therefore very limited.
In addition, the recovered energy can be used for such specific purposes as the preheating of furnace loads, the production of steam, and the preheating of combustion air by smoke. Furnace loads can be preheated only when the furnace operates for significant periods of time at maximum heat. Furnaces with alternating heat and hold cycles (or even cooling cycles) have a thermal output of smoke that is extremely variable and does not allow for maximum recovery of heat from the smoke. The same problem exists in the production of steam. For efficient recovery, steam consumption must be closely correlated with the thermal output of smoke; this is difficult to achieve. The preheating of combustion air by smoke is probably the most efficient use because a direct correlation between the smoke output and the combustion air flow rate is possible. Moreover, heat loss during transportation is greatly reduced. For optimum efficiency and minimum cost, a regular counterflow must be maintained between the two gases which exchange the heat. This counterflow can be either direct or indirect.
There are several direct counterflow heat exchangers currently in use. For example, metal tubular exhcangers with actual counterflow or crossflow are well known to those skilled in the art. These exchangers, however, have several disadvantages which include high cost, cleaning difficulties, and sensitivity to corrosive smoke components such as sulphur and vanadium or treatment salts. Racing of the furnace can also cause a heat surge and thereby destroy the furnace by overheating. Glass tube exchangers are generally corrosion resistant, but they are very fragile and costly. The tubes can also be destroyed by overheating or thermal shock. Normally, they are used only in cold climates as inserts for metal tubular exchangers to minimize the damage of acid condensation from the smoke. Metal plate exchangers can also be used. These exchangers are relatively inexpensive but they are sensitive to corrosion and are very difficult to clean because the gaps between the plates often measure only a few millimeters. In addition, the efficiency of a metal plate exchanger is greatly reduced when the plate is dirty or clogged. These exchangers can also be destroyed by overheating.
Indirect counterflow exchangers include two types-rotary exchangers and reversing cycle exchangers. Rotary exchangers have a metal or a porous ceramic wheel so that when the wheel is rotating, smoke flowing in one direction and combustion air flowing in the opposite direction alternately pass through each element of the wheel, parallel to the axis of rotation.
Although these heat exchangers are very efficient, they have low tolerance to dirt buildup, and therefore require clean and non-corrosive smoke. They are also highly sensitive to overheating even if such overheating is only temporary. In particular, smoke temperatures must not exceed 800.degree. C. and for this reason, manufacturers provide a cool air inlet to cool the smoke and to maintain it below the 800.degree. C. level. In addition, the outer part of the wheel generally wears out rather quickly. Because of the design of a rotary exchanger, a tight seal between the two gaseous streams is often poor. The fluid to be heated can leak into the fluid to be cooled and can account for 5% to 10% of the energy output. Consequently, the efficiency of the exchanger is further reduced.
Reversing cycle exchangers are composed of two parallel stationary units. When one unit is storing heat from smoke flowing in one direction, the other unit releases the heat it has stored to the gas to be heated which is flowing in the opposite direction. The flows of smoke and gas to be heated are periodically reversed. A Frankl exchanger is a reversing type that can be used at low temperatures. Each unit includes coils of corrugated metal sheets; these coils are separated from one another by fireproof spacers. This type of exchanger is used in cryogenics liquifying and separating gases. A brick wall exchanger is suitable for use at high temperatures. Each unit is composed of brick walls separating the gas flow channels. These exchangers have a low sensitivity to corrosion and occasional overheating. Yet, they are hard to clean and generally their cleaning destroys the exchanger. In addition, the investment cost is considerable because of the low ratio of "exchanger surface area to refractory volume" which requires an enormous exchanger volume. For the same reason, the reversing cycle is extremely long and can last for several hours.
The device of the present invention fulfills a long-felt need in the elimination of the aforementioned disadvantages in the recovery of waste heat from furnace smoke.